I do the previous-level vocab lessons first, otherwise they become leeches for me. (Because once I start on new-level radicals and kanji, my focus shifts…)
For the first 10 levels I didn’t have a strategy and did massive batches of lessons at once (terrible idea if you don’t want days with 400 reviews and others with 50) 
Now I usually do 15-25 lessons per day. Depending on how many previous-level vocab lessons remain, I’ll also be able to do new-level radicals on the same or next day. Radicals are easier than the rest (especially after level 14), so sometimes I’ll go over my daily lesson limit a bit to squeeze them in.
I’ve also started trying to use [Userscript] WaniKani Lesson Filter to mix up kanji and vocab lessons on new-level items, instead of doing all the kanji then all the vocab (the default). I’m hoping this will help me remember low-apprentice kanji readings better. 
My level-up time varies between 7.5 and 9 days, mostly depending on how many errors I make and at what stage. I try really hard to limit any errors once the second batch of kanji is unlocked, to keep my level timing short. I use [Userscript] Self-Study Quiz an hour or two after new lessons because the first interval is too long for me.